The present invention relates, in general, to wet flue gas desulfurization absorber towers, and more particularly, to an assembly for facilitating the attachment of a metal hood to a concrete shell of a spray absorber tower, and compensating for relative radial thermal expansion of the hood and the shell.
Sulfur oxides are produced in significant quantity by the combustion of coal or fuel oil, and the most common flue gas desulfurization process used with coal and oil-fired electrical generating power plants is known as “wet scrubbing.” For purposes of removing sulfur dioxide from flue gases, the cleansing fluid is typically an alkaline slurry which is sprayed into the flue gas as it flows through the absorber tower. Wet flue gas desulfurization processes for removing sulfur dioxide from flue gases involve the use of calcium-based slurries, or sodium-based or ammonia-based solutions. In this process the sulfur dioxide-containing flue gas is scrubbed with the calcium-based alkaline slurry or slurry reagent which may also include any number of additives to enhance removal, control chemistry, and reduce chemical scale. The slurry reagent, when contacted by sulfur dioxide, results in the absorption of the sulfur dioxide by the slurry and forms sulfites which are collected in a reaction tank situated below or forming the bottom of the absorber tower. Thereafter, the slurry can be oxidized to cause the alkali to react with the absorbed sulfur dioxide to yield a useful product. For example, in the case of desulfurization where a calcium-based alkaline slurry is used to absorb sulfur dioxide, an oxygen containing gas such as air is injected into the slurry to oxidize the aqueous sulfite into sulfate; the latter will then react with calcium ions in the slurry to form gypsum, a marketable product. It should be noted that the above reaction is exemplary, and that the teachings of this invention are not limited to the use of calcium-based slurries in a desulfurization reaction.
The technology for wet scrubbing provides gas-liquid contact in a number of differently configured systems. In recent years, wet flue gas desulfurization of the type commonly referred to as the in-situ forced oxidation type have been the preferred systems for achieving oxidation. These systems comprise two major components: the spray absorber in which the actual flue gas scrubbing takes place, and the reaction tank to allow for efficient utilization of the reagent. The absorber tower is fitted with a hood which forms the roof of the tower and defines the flue gas outlet opening from the tower. The majority of such systems are single loop systems in which the absorber and the reaction tank are combined to form a single structure. Some oxidation of sulfite to sulfate inevitably occurs in the gas-liquid contact zone of the absorber, and is referred to as natural oxidation so as to distinguish it from forced oxidation in which air is sparged through the slurry in the reaction tank. The sulfites must be oxidized to sulfates in order to maintain the reaction tank generally free of scale.
Due to chemical attack and the corrosive nature of the flue gas desulfurization slurries, the spray absorber tower has traditionally been constructed of either expensive corrosion resistant metal alloys or of carbon steel which is relatively inexpensive but susceptible to corrosion and chemical attack without the use of a corrosion resistant inner liner. Liners are usually made of rubber, fiberglass, or wall paper alloys to protect the carbon steel from the corrosive action of the chemicals inside the spray absorber and reaction tank. Traditionally, the tower shell and hood have been constructed of the same metal alloy. The absorber tower and the hood experience nearly the same temperature, due to the relatively high thermal conductivity of the typical metal alloys used in their fabrication, thus allowing the hood to be welded directly to the tower shell. The welding of the hood directly to the tower shell forms an integral structure with negligible differential thermal expansion occurring within the hood and tower structure itself. The hood-to-tower shell weld insures a gas tight seal at that junction.
Advancing technology has led to larger size absorber towers, with towers measuring approximately 60 feet in diameter currently in use. The present invention provides a cost-effective alternative to these large diameter metal towers, by fabricating the shell portion of the spray absorber tower from concrete, with an inner wall lining of corrosion resistant protective tiles. On the hand, it has been determined that hoods made of concrete for large diameter absorber towers are not cost-effective, so that it is desirable to make the hood from a corrosion resistant metal alloy. However, because of the difference in thermal expansion characteristics between the metal hood and the concrete shell, there is a need to provide an attachment assembly which will accommodate the relative radial thermal expansion of the metal alloy hood and the corrosion resistant concrete tower.